Northern Blot Background

Does anyone have information about eliminating non-specific hybridization to
rRNA on northern blots when using plasmid-derived [32]P-labeled probes? The
blot was blocked with denatured salmon sperm DNA prior to hybridization with a
plasmid probe having a GC content of approximately 56%.
Scott Bearden
swb at seqanal.mi.uky.edu